Glycine is a valuable compound of wide use as food additives for processed foodstuffs and raw materials for agricultural chemicals and medicines.
Among conventionally known preparation processes of glycine are predominantly the amination process of monochloroacetic acid, the Strecker process, the hydantoin process, etc. Although the monochloroacetic acid process is advantageous in that it does not employ toxic hydrogen cyanide, it byproduces secondary and tertiary amine homologues, resulting in low glycine yields. Further, in the Strecker process, it is necessary to separate iminodiacetic acid salts byproduced upon the reaction or neutralized salts after hydrolysis.
The hydantoin process produces glycine by the hydrolysis of hydantoin obtained from hydrogen cyanide and formaldehyde used as starting materials. For example, hydrogen cyanide, the aldehyde, ammonia and carbon dioxide are heated at not lower than 100.degree. C. in aqueous solvents (U.S. Pat. No. 3,536,726). In other approaches of the process, hydrogen cyanide, formaldehyde and ammonium carbonate are heated in an aqueous medium, or glycolonitrile, ammonia and carbon dioxide are reacted by heating followed by the removal of unreacted ammonia and carbon dioxide and the subsequent treatment with an alkylamine or alkyl ammonium hydroxide. In further approaches, glycolonitrile, ammonia and carbon dioxide are reacted by heating followed by the removal of unreacted ammonia and carbon dioxide and the subsequent treatment with a mineral acid for hydrolysis, or glycolonitrile, ammonia and carbon dioxide are reacted by heating followed by the treatment with an alkaline material such as the hydroxides, carbonates and bicarbonates of alkali metals or alkaline earth metals.
However, all of these processes produce byproducts in considerable amounts so that their glycine selectivities can not be said satisfactory. In the process in which hydantoins are hydrolyzed by a alkali salt or metal, the selectivity of glycine is improved, but the alkali is needed in molar equivalent or more to the glycolonitrile charged and moreover the glycine salt needs to be hydrolyzed. Further, the complicated separation step of glycine from byproduced salts, such as sodium sulfate and sodium chloride, not only deteriorates greatly the economical efficiency of the glycine production but also raises problems in treating the salts. In the industrial execution of the process, glycine must be isolated from the solution as crystals, in which case the glycine yield is inevitably lower than the one-pass glycine yield.
The foregoing U.S. Pat. No. 3,536,726 discloses a process in which glycine is produced via hydantoin without formation of such byproduced salts and most of the glycine is crystallized out from the reaction solution, the residual solution from which the crystallized glycine has been separated (referred simply to as the mother liquor hereunder) being recycled to the reaction zone (hereinafter simply referred to as the mother liquor recycle process). However, further improvement in the yield of glycine and reduction in the amount of byproducts are desired.
The present inventors have made intensive investigations into a process whereby glycine yield is improved in the production of glycine based on the hydantoin process. As a result, it has been found that by first subjecting glycolonitrile to a pre-reaction at a lower temperature than the main reaction temperature to convert it to another compound which is more stable than glycolonitrile and subsequently subjecting the compound to the main reaction at the predetermined higher temperature, the yields of glycine and products convertible to glycine are greatly improved. The present invention has been completed on the basis of this discovery.